Safe Yield of Groundwater Utilization for Domestic, Livestock and Hotels Requirement as An Anticipation of Sea Water Intrusion in The Coastal of Batang Regency

As a result of the groundwater utilization which exceeds its safe yield, the phenomenon of seawater intrusion is often found in some coastal areas. For this reason, the utilization of groundwater must not exceed its safe yield. This study aims to calculate the availability of groundwater and its safe yield and evaluate its utilization which does not exceed the safe yield. To achieve this purposes, groundwater availability is calculated using the static method by multiplying the area by the thickness of the aquifer and the specific yield. Utilization of groundwater for domestic, livestock and hotel requirement is carried out by multiplying the water demand for each of these sectors by the amount. Safe yield is calculated based on area width, groundwater fluctuation and its specific yield. The groundwater resources potential of research area amounted to 648,960,234 m3/year, while the safe yield was 127,702,334 m3/year. Utilization of groundwater for domestic, livestock and hotel requirement is 23,062,023 m3/year, so that the use of groundwater to meet domestic, livestock and hotel reqirement has not endangered the availability of groundwater. However, for Batang District, groundwater utilization has almost reached half of its safe yield.


Introduction
From time to time, human requirement in various aspects are increasing, including the requirement for water sources.Currently there are more and more problems related to the demand for and supply of water resources.What is desired is the existence of water resources that are sufficient in number, of good quality and evenly distributed in space and time.
One of the most important sources of water to support everyday human life is groundwater.Until now, groundwater is still the main source of water in various regions, especially for non-agricultural activities such as for domestic, livestock and hotels requirement.
Utilization of groundwater must not exceed its safe yield, which is a measure indicating the amount of groundwater that can be utilized from a groundwater basin without disturbing its aquifer conditions [1].According to [2], it is incorrect to assume that the safe yield is equivalent to the amount of groundwater recharge, because the safe yield is only a part of the groundwater recharge.Some other water will be lost from the aquifer in various ways.
As a result of the use of groundwater that exceeds its safe yield or other natural causes, the phenomenon of seawater intrusion is often found in some big cities [3].The phenomenon of seawater intrusion is often interpreted by the discovery of salt water in wells, which is caused by digging wells that penetrate the interface [4;5;6;7].The area below the interface is saltwater, while above it is fresh  .The stronger the pressure of fresh groundwater from the land on seawater, the interface will be more seaward or very deep, conversely if the groundwater pressure is reduced, seawater will push on land and the interface will be shallower [11;12;13].
Batang Regency is one of the regencies in Central Java Province which is located on the north coast of Java (Figure 1).Quoting a statement from the Director of PUDAM Sengan Kamulyan, Batang Regency, although there is not much exploitation of groundwater in this area, currently groundwater that tastes salty has been found in some coastal areas.This statement is also supported by the results of research by UGM Environmental Science Masters Field Work who found groundwater with an electrical conductivity value of more than 1500 µmhos/cm in several locations which indicated groundwater contained a high salt content.This finding is quite worrying because the industrial sector in Batang Regency is growing.If this problem is not anticipated as early as possible, the impact of seawater intrusion will become increasingly widespread.One of the forms of anticipation is to calculate the safe yield of groundwater utilization in the coastal area of this regency so that its utilization does not exceed the safe yield.The calculation results can be used as a basis for overall development planning.

Calculation of Groundwater Availability
The availability of groundwater referred to in this study is the amount of groundwater available per capita in annual average figures.Groundwater availability is calculated using a static approach using the formula: where Vat is the volume of groundwater that can be extracted from the aquifer, Sy is the specific yield or the percentage of water that can extracted from the aquifer and Vak is the volume of the aquifer, namely the area of the district multiplied by the thickness of the aquifer.The value of Sy is determined using the Sy table based on the type of rock material that makes up the aquifer, while the thickness of the aquifer is determined based on bor drilling data found in the study area.

Calculation of Safe Yield
In unconfined aquifers, safe yields of groundwater can be shown by groundwater fluctuations, aquifer area and specific yield, and calculated by the equation: Safe Yield = F .A .Sy where F is the fluctuation of the groundwater level obtained through interviews with residents and A is the area of the aquifer obtained from data on the area of the administrative area.

Deternination of Water Requirement
In this study, the water requirement that is taken into account is the demand for water sourced from groundwater, namely domestic, livestock and hotels.The requirement for water for domestic use is determined according to the size of the population and the amount of water requirement per capita per day.According to Mangku Sitepoe (1997 in [14], the requirement for water in big cities is generally > 150 liters/capita/day, in medium-sized cities it is 80-150 liters/capita/day, district cities are 60-80 liters/capita/day and village ranges from 30-60 liters/capita/day.Based on this criterion, because the research area is a regency that is developing towards a big city, the water requirement is determined to be 150 liters/capita/day. The water requirement for livestock is calculated based on the number of livestock and water consumption per per day, where the types of livestock that are calculated for water requirement are cows, buffaloes, horses (large livestock), goats, sheep, pigs (small livestock) and poultry.The standard water requirement for large livestock is 40 liters/day, small livestock is 5 liters/day and poultry is 0.6 liters/day [15].

Area and Boundary
The area of Batang Regency reaches 85.43 ha, but in this study it only includes districts located in coastal areas, namely Batang, Kandeman, Tulis, Subah, Banyuputih and Gringsing Districts with a total area of 33.43 ha.In terms of its territorial boundaries, it is bounded in the west by Pekalongan Regency, in the east by Kendal Regency, in the south by Warungasem, Wonotunggal, Bandar, Pecalungan, Limpung and Tersono Districts, while in the north by the Java Sea.

Climate
The climate conditions in the study area cannot be separated from the climatic conditions of Java Island, which are influenced by monsoon winds, namely global winds that blow due to the influence of the sun's position, so that it will always change direction every half year.During October to April the wet west monsoon blows.In that period, Java Island in general and the Coastal Regency of Batang in particular experienced the rainy season.On the other hand, from May to October, the monsoon winds do not bring moisture, so parts of Java experience a dry season.
Based on the classification of climate types from Koppen, the climate type in Pesisir Batang Regency is Am.In the Am type, the amount of rainfall that falls in the wet month can compensate for the short dry season.So even though the Am climate has one or two dry months, the rainfall in the other months is quite high, so the vegetation is not as affected by these short dry periods.In terms of the amount of rainfall, based on records in 2022 the highest rainfall will occur in February which reaches 510 mm and the lowest rainfall will occur in April by 10 mm.

Geology and Geohydrology
The coastal area of Batang Regency is composed of alluvial deposits, volcanic rocks and sedimentary rocks.Alluvial deposits that form the coastal plains and river bank plains consist of sand and clay.Sand deposits in coastal areas can be up to 50 meters thick.
Judging from its geohydrological conditions, the Coastal of Batang Regency is included in the Pekalongan-Pemalang groundwater basin and Subah groundwater basin areas.On the east, this groundwater basin is bounded by Kendal groundwater basin, on the west by Tegal-Brebes groundwater basin, on the south by Karangkobar groundwater basin and Wonosobo groundwater basin, while on the north by the Java Sea.According to Hendri Setiadi (in the Groundwater Basin Map of Central Java Province and Yogyakarta Special Region), the Pekalongan-Pemalang groundwater basin has an unconfined aquifer discharge of 644 million m 3 /year and a confined aquifer discharge of 17 million m 3 /year, while the Subah groundwater basin has an unconfined aquifer discharge of 427 million m 3 /year and a confined aquifer discharge of 8 million m 3 /year.

Groundwater Availability and Safe Yield
Availability of groundwater in the study area was obtained based on analysis per district which is located in the coastal area.Based on this analysis, the availability of groundwater in each district is determined by multiplying the aquifer thickness, specific yield and the area of each district.The safe yield of groundwater for utilization was calculated by the multiplication of the area, specific yield and groundwater fluctuations.The results of calculating these two parameters are shown in Taking into account Table 1, Subah District has the largest groundwater availability, reaching 308,981,478 m 3 /year, while Batang District has the smallest groundwater availability, amounting to 30,012,238 m 3 /year.Likewise, when viewed from the safe yield of utilization, Subah District also has the largest safe yield of 41,289,284 m 3 /year, while the smallest safe yield is in Batang District of 11,498,942 m 3 /year.

Groundwater Utilization
The calculation of groundwater utilization for domestic requirement is based on the number of residents in the districts located in coastal areas.In 2022, the population in the coastal area will be 387,455 people (Table 2).With the water requirement per person of 150 liters/day, the total water requirement is 58,118,250 liters/day.In terms of districts, Batang District has the largest domestic water requirement, namely 20,421,750 liters/day, while Banyuputih District has the smallest domestic water requirement, namely 5,597,250 liters/day.In the study area, there are many livestock businesses, both large livestock, small livestock and poultry (Table 3).Types of large livestock that are cultivated by residents include types of large livestock such as cows, buffaloes and horses, while types of small livestock that are cultivated are goats, sheep and pigs.For poultry, several types of poultry that are cultivated are domestic chicken, purebred chicken, ducks and geese.The calculation results of groundwater utilization for livestock requirement show that the water requirement for large livestock is 163,880 liters/day, small livestock is 213,800 liters/day and poultry is 4,655,753 liters/day, so the total water requirement for livestock is 5,033,433 liters/day .The calculation of groundwater utilization for hotel requirement is estimated based on the number of available beds and is assumed to be 50% filled.The amount of water requirement for hotel residents is considered to be the same as the water requirement for big city residents, namely 150 l/day.Based on data from Batang Regency and Figure 2023, it is known that the number of hotels in the districts located on the coast of Batang Regency are 10 hotels with a total of 394 bed rooms, so the total water requirement for hotels is 31,950 liters per day.The complete calculation is shown in Table 4.

Evaluation of Groundwater Utilization and Safe Yield
The calculation results show that the amount of groundwater utilization for domestic, livestock and hotels requirement in the study area is 23,062,023 m 3 /year, while the safe yield of groundwater extraction is 127,702,334 m 3 /year (Table 5).Based on these data it can be said that groundwater extraction which has been carried out by residents has not endangered the availability of groundwater, so there is no decrease in groundwater pressure which can cause seawater intrusion.The presence of salty water in a small part of the coastal area is probably caused by connate water, namely ancient seawater trapped on land in past geological times.Even so, Batang District needs to get attention because water withdrawal has almost reached half of the safe yield.

Conclusion
1) The availability of groundwater in the study area is 648,960,234 m 3 /year, with safe yield 127,702,334 m3/year.2) Utilization of groundwater for domestic , animal husbandry and hotels requirement in the research area 23,062,023 m3/year.3) In general, the use of groundwater to meet domestic, livestock and hotel requirement has not endangered the availability of groundwater, so there is no decrease in groundwater pressure which can lead to seawater intrusion.However, for Batang District, groundwater utilization has almost reached half of its safe production.

Table 1 Table 1 .
Groundwater availability and safe yield at each district in the study area

Table 2 .
Utilization of groundwater for domestic requirement

Table 3 .
Utilization of groundwater for livestock requirement

Table 4 .
Utilization of groundwater for hotel requirement

Table 5 .
The utilization of groundwater and safe yield